1. Field of the Invention
This invention relates to a device and method for the chemical deposition of synthetic material on an elongated substrate member.
2. Description of the Related Art
As is well known, an optical fibre is obtained by a process of drawing a preform of vitreous material. In particular the positioning of the preform in a vertical position within a furnace to cause the fusion of a lower part of the said preform is known. The molten material is then drawn downwards by a traction device obtaining a filiform member which forms the optical fibre.
Patent EP 367871 in the name of Corning Glass Works describes a method for obtaining a step index optical fibre with a rapid jump in the refractive index between the core and the cladding. This method initially comprises the step of depositing particles of glass comprising base glass and a refractive-index-increasing dopant onto a mandrel. The mandrel is then removed and the resulting soot preform is consolidated so as to form a core preform having a dopant-deficient surface region. The core preform is stretched and the hole in it is closed to form a core bait rod. Cladding glass soot is then deposited on the core bait rod in a density of at least 0.5 g/cc. This is achieved by directing the flame of the auxiliary burner onto the core bait rod immediately prior to depositing the cladding glass soot upon it. A final preform is thus obtained which is then consolidated and drawn so as to obtain an optical fibre.
A process of the type just described to produce a final preform is commonly known as an OVD (Outside Vapour Deposition) process.
A different method of producing a fibre preform is proposed by EP0041397(A1) in the name of Corning, this method comprising applying particulate material on one end of a starting member to form a coating thereon and longitudinally translating the coating while applying additional particulate material to form a preform body. In this way, the starting member is continuously removed from the preform body leaving a longitudinal aperture in the preform body. Means are provided for supporting, rotating and translating the preform as it is being formed, for example planetary drive wheels.
Again according to patent EP 367871, a traction device comprising a pair of traction driving wheels (indicated by 52 in FIG. 3 of patent EP 367871) which apply a downward traction force to the opposite sides of the said core preform is used to draw out the core preform.
The applicant has noted that, when the latter step is performed, it is possible that, as a result of inaccurate positioning and/or operation of the traction wheels, opposite sides of the core preform are subjected to different forces which cause bending of the core preform during the process of its formation; for this reason core preforms which are not perfectly straight, or which have shape defects, can be produced.
Again in accordance with patent EP 367871, before the cladding glass soot is deposited on the core preform (which at this stage comprises an elongated member of substrate), the core preform is attached at one end to a gripping device (indicated by 56 in FIG. 4 of patent EP 367871) mounted on a lathe. The applicant has noted that, when this step is carried out, core preform positioning errors can occur with the result that the axis of the core preform does not coincide with the intended axis of rotation.
The applicant has noted that the abovementioned positioning errors and the abovementioned shape defects can give rise to a non-negligible straightness error in the core preform during the last step of chemical deposition.
The applicant has therefore noted that these positioning errors and these shape defects can combine to create a non-homogeneous final preform (from which the optical fibre will be drawn), that is a preform having a central portion (defined by the core preform) which is curved and has a non-constant radial distance from the outer surfaces of the said preform; in other words, if a transverse cross section is taken across the preform, it can be seen that the central circular portion corresponding to the cross section of the core preform is not concentric with respect to the corresponding circular cross section of the final preform.
This concentricity error is maintained during the drawing step and the optical fibre produced therefore has a core which is not concentric with respect to the cladding.
The core/clad concentricity therefore provides a measure of how well the core axis is aligned with the cladding axis and is therefore a fundamental parameter for an optical fibre. More particularly, concentricity is defined as the distance between the axis of the core and the cladding. Typically the concentricity should have a small value (e.g. less than 0.5 μm, preferably less than 0.3 μm) so that when two end portions of two different optical fibres are coupled together the attenuation in the transmitted light is small. Optical fibres are in fact typically connected by aligning the outer surfaces of the corresponding claddings and therefore, if the cores are not perfectly located along the axes of the corresponding fibres, coupling between the two cores may be partial, giving rise to a coupling with high losses.
The problem of concentricity is also tackled by EP0630866(A1) in the name of Corning, which refers to an apparatus and a method for making an optical fiber prefom. This document teaches attaching a rod to an upper cane handle and a lower cane handle to make a cane assembly, and holding the cane assembly with upper and lower cane chucking assemblies. This apparatus is suitable to apply a tensile force along the longitudinal axis to the cane assembly during the deposition of soot thereon.